1. Technical Field
Example embodiments relate to a gasket, and a bipolar battery including at least one gasket. Example embodiments also relate to a method for manufacturing a gasket.
2. Description of Related Art
A bipolar battery construction comprises an electrically conductive bipolar layer (or “biplate”) that serves as an electrical interconnection between adjacent cells in the battery as well as a partition between the cells. In order for the bipolar construction to be successfully utilized, the biplate should be sufficiently electronically conductive to transmit current from cell to cell, chemically stable in the cell's environment, capable of making and maintaining good contact to the electrodes, capable of being electronically insulated from other biplates, and sealable around the boundaries of the cell so as to contain electrolyte in the cell.
The above characteristics are more difficult to achieve in secondary (rechargeable) batteries due to the charging potential that can generate gas inside the battery, and in alkaline batteries due to the creep nature of the electrolyte. Achieving the proper combination of these characteristics has proven very difficult.
A common type of battery design is the so called “flooded” battery wherein the electrolyte within the battery completely fills the porous spaces within the battery, with wet liquid electrolyte present in excess of that which can be absorbed by the constituent electrodes and separators within the battery. More recent battery designs are designated as “starved electrolyte” or “recombinant” batteries. In this sort of battery design, the porous spaces within the constituent electrodes and separators are not completely filled with electrolyte. Instead, some of this porous space is occupied by gases. As a result, the volume inside the battery surrounding the electrolyte is substantially dry due to the remaining potential for the capillary action of the porous spaces to absorb wet liquid electrolyte. This results in a battery configuration that is essentially damp, but not flooded or wet inside the battery. Such batteries have a volume into which gases generated within the battery can be contained. Starved electrolyte battery designs are typically much less tolerant of loss of electrolyte when compared to flooded batteries, as they have no extra wet reserve of electrolyte to compensate for electrolyte loss. Consequently, a starved electrolyte battery's internal volume is sealed from the ambient environment during normal use. It is common in the art to refer to starved electrolyte batteries as having a sealed configuration, as further described herein.
For maintenance-free operation it is desirable to operate rechargeable batteries in a sealed configuration. However, sealed bipolar designs typically utilize flat electrodes and stacked-cell constructions that present design challenges for proper containment of gases present and generated during cell operation. In a sealed construction, gases generated during charging should be chemically recombined within the cell for stable operation. The pressure-containment requirement creates additional challenges in the design of a stable bipolar configuration.
Technical fields such as transportation, communications, medical and power tools (for example) are generating specifications that existing batteries cannot meet. These include higher cycle life and the need for rapid and efficient recharges.
NiMH systems are seen as an alternative to meet cycle life specifications, but costs for existing conventional fabrication are too high.
In U.S. Pat. No. 5,344,723 to Bronoel et al., a bipolar battery is disclosed having a common gas chamber, which is created by providing an opening through the biplate (conductive support/separator). The opening is also provided with a hydrophobic barrier to prevent passage of electrolyte through the hole. Although a problem with pressure differences between the cells may be avoided, there is still a disadvantage with the described battery. The outer sealing around the edge of each biplate still has to be fluid-tight, which is very difficult to achieve. If the outer sealing is not fluid-tight, the electrolyte, contained in the separator between the electrodes and in the electrodes, may migrate and form a continuous ionic current leakage path from one cell to another.
In U.S. Pat. No. 5,441,824 to Rippel, a quasi-bipolar battery is disclosed where the structure of the battery attempts to address problems inherent when using the corrosive lead-acid chemical system in a bipolar configuration having biplates and separators. Here, the biplate edges are encapsulated within a gas-tight continuous compliant frame material. The separator edges are also similarly encapsulated in a gas-tight continuous complaint frame having gas passages formed into them. Such encapsulation processes are expensive and difficult to accomplish in a reliable, manufacturable fashion. The frame design disclosed by Rippel has comparatively large areas present along the sealing surfaces. As a result, large forces are needed to cause the required compressive strain in the disclosed frame necessary to induce a gas tight seal. This large force must be borne by the structure of the battery, resulting in larger size, higher weight and increased cost of the resulting battery. Rippel's disclosure also does not address the problem of ionic currents that may flow in the electrolyte present in the gas passages that can cause imbalanced self-discharge of individual bipolar electrodes in the battery.
The use of a common manifold for primary (non-rechargeable) reserve batteries to be activated in the field by filling of the electrolyte immediately prior to use is well known in the art. In U.S. Pat. No. 4,626,481 to Wilson discloses a flooded bipolar battery design using a primary reserve activated Li/SOCl2 system. This design comprises a frame encapsulating a biplate. This disclosure refers to this frame as an “insulating layer”. Again, such a continuous encapsulation is expensive and more difficult to manufacture. Wilson addresses the problem of reducing ionic currents that may flow in the electrolyte present in the gas passages by teaching use of a low conductivity electrolyte, which is clearly undesirable in a starved electrolyte secondary battery when high power density is desired.
In the published international patent application WO 03/026042 A1, assigned to the present applicant, a hydrophobic barrier is introduced around the electrodes instead of around the opening in the biplate (as disclosed in U.S. Pat. No. 5,344,723). A pressure relief valve is also introduced to prevent a pressure build up inside the case. It is however rather expensive to manufacture a bipolar battery of this design in large quantities and therefore there is a need to construct a new bipolar battery having a smaller quantity of components, using simpler processing techniques to manufacture a bipolar battery.
In the published international patent application WO 2005/048390 A1, assigned to the present applicant, a bipolar battery design is disclosed. The bipolar battery has a gasket made from a hydrophobic material with a built-in gas passage arranged between adjacent biplates, wherein the gas passages within the gaskets create a common gas space within the battery and at the same time electrolyte is prevented from migrating between cells. However, a high degree of mechanical preloaded force should be maintained over the gaskets to achieve these objectives, which in turn requires an outer casing that may withstand the stress that will result from the force needed. Teaching of details regarding top level construction resulting in a finished starved electrolyte bipolar battery is found in this published international patent application WO 2005/048390 A1, which is incorporated here by reference.